Since 1987, twenty-one International Workshops on the Methodology of Twin and Family Studies have been held in Leuven, Belgium and Boulder, Colorado. We propose to hold a scientific meeting 'The Twenty-First Anniversary International Workshop on the Methodology of Twin and Family Studies' in the summer of 2008 in Leuven, Belgium on the twenty-first anniversary of the inaugural workshop. This will be an opportune time to assess the effectiveness of past workshops and to plan the content and format of future workshops. The primary objectives of the anniversary workshop are to: 1) bring together the faculty who have over the past 20 years contributed to at least 3 workshops and who are experts in training of statistical genetic methodology; 2) provide state-of- the-art presentations (which may include hands-on practical components) by world leaders in data analysis. Topics would range from assessment and phenotyping, to basic and advanced genetic epidemiology, linkage and association studies, and the latest advances in haplotype mapping and whole genome association studies; 3) present a symposium on 'the past, present and future' of statistical genetics; 4) promote discussions between the experts of the different specialty areas and 5) stimulate new collaborations. The workshop faculty members are leaders in the development of new statistical methods, which are required to address the increase in quantity and complexity of data being collected in genetically informative studies. The anniversary workshop will focus on novel research methods. These factors will provide both faculty and researchers a strong incentive to participate. A large number of studies are collecting large amounts of high-quality data on genetic and environmental factors that may be responsible for susceptibility to common health problems such as obesity, psychiatric disorders and substance abuse. The statistical methods required to unravel these factors are becoming increasingly complex and researchers in a wide variety of disciplines need training in order to make the most of these data sets. This workshop will provide that training and identify promising areas for future didactic and collaborative meetings. [unreadable] [unreadable] [unreadable]